1. Field of Invention
The invention relates generally to the field of oil and gas production. More specifically, the present invention relates to a modular apparatus for providing communication between members of a downhole string. Yet more specifically, the present invention relates to a cartridge inserted into an end of a perforating gun equipped with a receptacle or contact at both ends for connection to a signal line through a perforating gun string.
2. Description of Prior Art
Perforating systems are used for the purpose, among others, of making hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined zones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically completed by coaxially inserting a pipe or casing into the wellbore. The casing is retained in the wellbore by pumping cement into the annular space between the wellbore and the casing. The cemented casing is provided in the wellbore for the specific purpose of hydraulically isolating from each other the various earth formations penetrated by the wellbore.
Perforating systems typically comprise one or more perforating guns strung together, these strings of guns can sometimes surpass a thousand feet of perforating length, but typically shorter in a wireline application. In FIG. 1 an example of a prior art perforating system 10 is shown disposed in a wellbore 12 and made up of a string of perforating guns 14 connected in series. Typically, subs 15 may connect adjacent guns to one another. The perforating system 10 is deployed from a wireline 16 that spools from a service truck 18 shown on the surface 20. Generally, the wireline 16 provides a raising and lowering means as well as communication and control connectivity between the truck 18 and the perforating system 10. The wireline 16 is threaded through pulleys 22 supported above the wellbore 12. In some instances, derricks, slips and other similar systems are used in lieu of a surface truck for inserting and retrieving the perforating system into and from a wellbore. Moreover, perforating systems may also be disposed into a wellbore via tubing, drill pipe, slick line, coiled tubing, to mention a few.
Included with each perforating gun 14 are shaped charges 24 that typically include a housing, a liner, and a quantity of high explosive inserted between the liner and the housing. When the high explosive in a shaped charge 24 is detonated, the force of the detonation collapses the liner and ejects it from one end of the shaped charge 24 at very high velocity in a pattern called a “jet” 26. The jet 26 perforates casing 28 that lines the wellbore 12 and cement 30 and creates a perforation 32 that extends into the surrounding formation 34.
Shown in FIGS. 2A-D are sectional views of the prior art perforating gun 14 of FIG. 1. As shown, the shaped charges 24 are typically connected to a detonating cord 36, which when detonated creates a compressive pressure wave along its length that initiates detonation of the shaped charges 24. A detonator 38 is typically used to set off detonation within the detonation cord 36. In FIG. 1, the detonator 38 is shown in a firing head 40 provided in the string of perforating guns 14. Initiating detonation of the detonation cord 36 generally takes place by first sending an electrical signal from surface 20 to the detonator 38 via the wireline 16. Referring back to FIGS. 2A-D, an upper connection sub 42 contains a terminal 44 for receiving signals transmitted along the wireline 16. A signal line 46 attaches to the terminal 44 and conveys signal(s) from the wireline 16 to the remaining portions of the perforating system 10, including the detonator 38. Multiple connectors 48 are used to make up the signal line 46 through the successive connecting subs 15 and perforating guns 14. The signal through the signal line 46 initiates high explosive in the detonator 38 that transfers to the attached detonation cord 36. Detonators 38 may sometimes be provided within connecting subs 15 for transferring the detonating charge along the entire string of perforating guns 14. Without proper continuity between the wireline 16 and detonator(s) 38, the shaped charges 24 cannot be detonated. However, failure points in the signal line 46 are introduced with each connector 48.
Generally the detonators are connected to the detonating cords in the field just prior to use. Thus they are shipped to the field with the electrical portions and high explosive coupled together in a single unit. Because of the risks posed by the high explosives and the threat of a transient electrical signal, shipment and storage of the detonators is highly regulated, this is especially so when being shipped to foreign locations. Additional problems may be encountered in the field when connecting detonators to the detonating cord. Perforating guns when delivered to the field generally have the shaped charges and detonating cord installed; to facilitate detonator connection some extra length of detonating cord is provided within the gun. Connecting the detonator to the detonating cord involves retrieving the free end of the detonating cord and cutting it to a desired length then connecting, usually by crimping, the detonator to the detonating cord. These final steps can be problematic during inclement weather. Additionally, these final steps fully load a perforating gun and thus pose a threat to personnel in the vicinity. Accordingly benefits may be realized by reducing shipping and storage concerns, increasing technician safety, and minimizing the time required to finalize gun assembly in the field.